Adipogenesis is a complex process that is highly regulated by positive and negative stimuli, including a variety of hormones and nutritional signals (Gregoire et al., Physiol. Rev., 78:783-809 (1998); Cowherd et al., Semin. Cell Dev. Biol., 10:3-10 (1999); Rangwala et al., Annu. Rev. Nutr., 20:535-559 (2000); Rosen et al., Genes Dev., 14:1293-1307 (2000); Gregoire, Exp. Biol. Med. (Maywood), 226:997-1002 (2001); Koutnikova et al., Ann. Med., 33:556-561 (2001); Camp et al., Trends Mol. Med., 8:442-447 (2002); MacDougald et al., Trends Endocrinol. Metab., 13:5-11 (2002)). At the cellular and molecular levels, parts of the program of adipogenesis has been relatively well delineated. Preconfluent preadipocytes undergo proliferation and growth arrest, but when triggered by proper stimuli, undergo clonal expansion leading to a second growth arrest followed by terminal differentiation (Gregoire et al., Physiol. Rev., 78:783-809 (1998); Cowherd et al., Semin. Cell Dev. Biol., 10:3-10 (1999)). The latter part of this process is under a complex series of transcriptional controls involving CCAAT/enhancer-binding protein (C/EBP) β, δ, α, peroxisome proliferator-activated receptor (PPAR)γ, the signal transducers and activators of transcription (STAT)-1, -3, -5, and others (Rosen et al., Genes Dev., 14:1293-1307 (2000); Aubert et al., Cell. Mol. Life Sci., 56:538-542 (1999); Stephens et al., J. Biol. Chem., 271:10441-10444 (1996); Harp et al., Biochem. Biophys. Res. Commun., 281:907-912 (2001)). This precedes the synthesis of proteins characteristic of a fully differentiated phenotype, such as fatty acid synthase (FAS) and glucose transporter-4 (Glut-4, official gene symbol is Slc2a4). More recently, PPARγ coactivator-1 (known as PGC-1α or Ppargc1a) has been identified as a potential unique regulator leading to brown fat differentiation, involving the induction of uncoupling protein (UCP)-1 expression (Puigserver et al., Cell, 92:829-839 (1998)).
Less is known about the events prior to the initiation of this transcriptional cascade during which preadipocytes are released from suppressions and become committed to terminal differentiation (Gregoire et al., Physiol. Rev., 78:783-809 (1998); MacDougald et al., Trends Endocrinol. Metab., 13:5-11 (2002)). Some of the known inhibitors of the preadipocyte-adipocyte transition for white fat include the Wnt family of proteins (Ross et al., Science, 289:950-953 (2000)), preadipocyte factor-1 (Pref-1, also known as Dlk1) (Smas et al., Cell, 73:725-734 (1993)), Gata3 (Tong et al., Science, 290:134-138 (2000)), and the retinoblastoma (pRb) family of proteins (Chen et al., Genes. Dev., 10:2794-2804 (1996)). Very little is known about whether similar mechanisms play a role in brown adipocyte differentiation, or which factors and mechanisms regulate the production and function of these inhibitors.
Both insulin and IGF-1 have been shown to exert effects on adipocyte differentiation in vivo and in vitro (Gregoire et al., Physiol. Rev., 78:783-809 (1998); MacDougald et al., Trends Endocrinol. Metab., 13:5-11 (2002)). These factors utilize a complex signalling pathway to exert their pleiotropic biological effects involving activation of their respective cell surface receptors and phosphorylation of several intracellular insulin/IGF-1 receptor substrates (IRS). Brown preadipocytes lacking insulin receptors fail to differentiate (Entingh et al., J. Biol. Chem., 278:33377-33383 (2003)), and cells lacking IRS proteins show a range of differential defects, from normal differentiation in IRS-4 knockout (KO) cells to severely impaired adipogenesis in IRS-1 KO preadipocytes (Tseng et al., Mol. Cell. Biol., 24:1918-1929 (2004)).
Necdin is a growth suppressor that is expressed in virtually all postinitotic neurons in the brain. See, e.g., OMIM entry No. *602117. The necdin polypeptide sequence is given at Genbank Acc. No. Q99608. The homo sapiens necdin gene, promoter and 5′UTR region are given at Genbank Acc. No. AF000113.1.